Automobile bodies have conventionally been treated with zinc phosphate as bed treatment in order to improve corrosion resistance and adhesion. Since a zinc phosphate treating agent used for chemical conversion treatment contains therein much phosphorus or nitrogen and also contains much heavy metals such as nickel and manganese for improving the performance of the resulting chemical film, its adverse effect on the environment cannot be neglected and further, this chemical conversion treatment has a problem of industrial waste disposal owing to a large amount of sludge of zinc phosphate and iron phosphate remaining after the treatment.
As a corrosion-preventive primer coating for automobile bodies, an electrodeposition coating composition containing an aqueous dispersion of a water dispersible and electrodepositable cationic resin and a water soluble salt of zinc is proposed, as disclosed in Japanese Laid-Open Patent Publication No. 7-166111 (1995). This electrodeposition coating composition containing a water soluble salt of zinc such as zinc lactate or zinc acetate has problems in the stability of the coating composition and finish appearance.
An invention relating to an aqueous solution for metal surface treatment containing from about 0.001 to 1.0 wt. % of a metal ion selected from the group consisting of titanium ion, zirconium ion and hafnium ion and mixtures thereof, a polymer having a specific structural formula, acid salt thereof, or a mixture thereof is already disclosed in Japanese Laid-Open Patent Publication No. 59-207972 (1984).
Also an invention relating to a composition containing an aqueous solution of a specific polymer material or copolymer compound, and a solution or dispersion of a compound of titanium, zirconium, hafnium or silicon is disclosed in Japanese Laid-Open Patent Publication No. 2-608 (1990).
In electrodeposition coating compositions, when voltage is applied to a metal article to be coated after dipping it in the coating composition, charges transfer from the electrodeposition coating composition to the article and precipitation occurs. The wet film thus precipitated is then baked and dried, followed by fusion of the film to form a continuous film. During precipitation of the film, however, existence of foreign ions (such as Na+ and PO3−) causes abnormal precipitation or disturbs smooth fusion of the film because of an abnormally concentrated current flow to a portion of the film, which sometimes leads to deterioration in finish appearance. For example, single use of the metal ion or polymer composition as described in Japanese Laid-Open Patent Publication No. 59-207972 or Japanese Laid-Open Patent Publication No. 2-608 (1990) for an electrodeposition coating composition causes aggregation in the coating composition, thereby impairing the stability of the coating composition or deteriorating the finish appearance of the coating.